The present invention relates to a method and an apparatus for drilling the ground, wherein the actual energy required by the drilling process is calculated by measuring drilling parameters. Said method is applicable, in particular, to the installation of foundation piles in the ground.
In particular, the present invention relates to a method and an apparatus for the installation of piles in the ground in accordance with the CAP (Cased Augered Piles) and CSP (Cased Secant Piles) technologies. These technologies employ an excavation equipment consisting of a continuous ground drilling propeller housed inside a covering tube, the main purpose of which is to stabilize the borehole walls and to allow advancement into particularly hard ground.
In general, said equipment comprises a drilling machine like the one shown in FIG. 1, which is typically tracked and comprises a drilling turret 2 with a vertical guiding antenna and at least one rotary head for driving drilling batteries, which are slideable along the antenna and are driven by a mechanism (generally a rope mechanism) capable of exerting a pull and a thrust onto the batteries.
First inner drive means 9 are connected to a propeller battery 5, and second independent external drive means 10 are connected to tube 7, for the purpose of selectively rotating the two elements. In the most common configuration, the inner drive means 9 belong to a higher first rotary head 4, whereas the external drive means 10 are connected to a lower second rotary head 6. The propeller and the tube are preferably rotated in a discordant manner, preferably the tube rotating counterclockwise and the propeller rotating clockwise. The propeller and the tube can move axially relative to each other by means of suitable selective drive means.
The main function of the covering tube is to stabilize the borehole walls and to allow advancement into particularly hard ground while maintaining a high degree of borehole verticality. The covering hole is also fitted with excavating teeth at the bottom edge to bite the ground in the annular excavation portion. The rotations must advantageously occur in opposite directions to allow the ground to climb up along the propeller because of the inner friction between the material and the tube wall, to be then discharged in the upper part of the covering tube.
The excavation equipment further comprises a driving cab 8 for an operator, which includes all the manipulators for operating the machine and the systems for displaying and controlling the most important parameters.
It is known that installing foundation piles in the ground, and in particular drilling the holes for such piles, is easier when the type of ground involved is known. One possible way of knowing the characteristics of the ground is to make preliminary drillings in the excavation site. This procedure is not always compatible with the time available. Moreover, making a preliminary drilling for each excavation would imply significantly higher costs. It is therefore necessary to be able to deduce the characteristics of the ground while the actual drilling work is being carried out. Thanks to the sensors available on the operating machine, it is possible to make real-time analyses to interpret the characteristics of the ground being drilled.
Patent EP1942247 describes a method and a device for compaction drilling, wherein a drilling tool is fed into the ground through the effect of the thrust and rotation generated by suitable means.
The operating drilling parameters (torque, revolution speed, descent speed per revolution, constituting the alpha and thrust parameters) are measured during the drilling process and are simultaneously sent to a computer, which correlates them mathematically to determine the load capacity of an “ideal” pile cast into the borehole just drilled. In order to evaluate the load capacity of the borehole, the system calculates the load capacity characteristic, called α, and enters it into a previously prepared table containing data relating to drilling tests previously carried out in grounds having known characteristics. This ground type estimation also allows establishing the final depth of the pile to be made as a function of the measured values.